This invention is in the field of wireless communications, and is more specifically directed to the digital modulation of broadband signals in such communications.
The popularity of mobile wireless communications has increased dramatically over recent years. It is expected that this technology will become even more popular in the foreseeable future, both in modem urban settings and also in rural or developing regions that are not well served by line-based telephone systems. This increasing wireless traffic strains the available communications bandwidth for a given level of system infrastructure. As a result, there is substantial interest in increasing bandwidth utilization of wireless communications system to handle this growth in traffic.
Modern digital communications technology utilizes multiple-access techniques to increase bandwidth utilization, and thus to carry more wireless traffic. Under current approaches, both time division multiple access (TDMA) and code division multiple access (CDMA) techniques are used in the art to enable the simultaneous operation of multiple communications conversations, or wireless “connections”. For purposes of this description, the term “conversations” refers to either voice communications, data communications, or any type of digital communications. As evident from the name, TDMA communications are performed by the assignment of time slots to each of multiple communications, with each conversation transmitted alternately over short time periods. CDMA technology, on the other hand, permits multiple communications sessions to be transmitted simultaneously in both time and frequency, by modulating the signal with a specified code. On receipt, application of the code will recover the corresponding conversation, to the exclusion of the other simultaneously received conversations.
This trend toward heavier usage of wireless technologies for communications, in combination with the advent of so-called third-generation, or “3G”, wireless communications to carry not only voice, but also data, video, and other high data rate payloads, will require continuing improvements in the processing capabilities of the communications equipment. In particular, the higher required data rates will require corresponding increases in the digital processing of the communications payloads.
The process of digital modulation of a signal to be transmitted is of particular importance in the digital processing of broadband, or spread-spectrum signals. As known in the art, conventional wireless transmission is carried out according to quadrature amplitude modulation (QAM), in which each modulated symbol is represented by the combination of an amplitude value and a phase rotation (the phase rotation being relative to a previous symbol). The number of bits in the data symbol being modulated determines the density of the QAM “constellation”; for example, QAM modulation of an eight bit data symbol involves a 256-point QAM constellation, in which 256 combinations of amplitude and phase are used to represent each of the 256 (28) possible data values.
Each data symbol to be transmitted must therefore be converted into the appropriate amplitude and phase modulation point. According to conventional QAM systems, this modulation is carried out by considering the data value as the combination of an in-phase (I) digital word and a quadrature (Q) digital word. The I and Q values represent the real and imaginary parts of a complex value, so that their combination represents a point in the complex plane, involving both amplitude and phase.
FIG. 1 illustrates the data flow for a transmitting element, such as a wireless handset or base-station, according to conventional techniques. In this broadband example, an input data bitstream has been spread into multiple “chips”, as known in the art, such that each bit of the input bitstream consists of a series of samples (the “chips”) that are modulated by a spreading code. This spread data stream is represented as in-phase component I(k) and a quadrature component Q(k). As shown in FIG. 1, multiplier 3 effectively shifts each digital word in the sequence of quadrature component Q(k) by 90° (indicated by multiplication by square root of−1, represented in the art as imaginary operator “j”). Adder 2 then combines this phase-shifted quadrature component jQ(k) with its corresponding digital word in the sequence of in-phase component I(k).
The combined I and Q components from adder 2 are then scrambled by a scrambling code c(k) prior to its transmission. As conventional in the cellular telephone art, scrambling code c(k) is cell-specific in the downlink case, in that all transmissions from a central office that take place in the same physical cell use the same scrambling code. Scrambling code c(k) thus allows each remote system element to resolve incoming communications for its cell from those that may be received from other cells. Conversely, in the uplink case, the scrambling code c(k) is user-specific, dedicated to the particular transmitting wireless unit. Typically, scrambling code c(k) is a “long” code, for example 4096 chips in length. According to this conventional example of FIG. 1, the scrambling code c(k) also includes both an in-phase component Ic(k) and a quadrature component Qc(k). Similarly as for the data bitstream, in-phase scrambling code component Ic(k) is added, by adder 4, with quadrature scrambling code component Qc(k) after application of a 90° phase-shift by multiplier 5.
In this conventional example, the combined in-phase and quadrature data signal from adder 2 is mixed with the combined in-phase and quadrature scrambling code signal from adder 4, at mixer 6. In the digital context, mixer 6 is a complex multiplier function or circuit. Signal Y(k), at the output of mixer 6, is the complex modulated output of these operations, and includes in-phase and quadrature components. These components are then filtered and used to modulate in-phase (cosine wave) and quadrature (sine wave) analog signals at the appropriate carrier frequency.
The operations of FIG. 1 are conventionally carried out by digital signal processing operations, such as may be carried out by a high performance digital signal processor (DSP), such as the TMS 320c5x or 320c6x families of digital signal processors available from Texas Instruments Incorporated. It has been observed, in connection with the present invention, that the complex multiplication of the data and scrambling codes, shown by mixer 6 in FIG. 1, is a particularly cumbersome operation, whether performed as a software routine by a DSP, or in dedicated logic hardware. It is contemplated that, especially as data rates increase, this operation can be a bottleneck in the transmission of wireless signals.